STUDIES OF SPORE-PLANTS 259 



The common molds (black, green, brown) make preserva- 

 tion of some foods for human use difficult, for as the experi- 

 ments have shown, brief exposure to the boiling temperature 

 does not kill the spores. 



During a rainy season the mildews form a white coating 

 on books, paper, clothing, carpets and other organic objects 

 in our homes. Other mildews form their mycelia on the 

 lower sides of many leaves, and some species cause serious 

 diseases of the plants. The mildews attacking grapes grow 

 on the leaves and fruit and cause enormous financial loss 

 in America and Europe. Mildews often destroy lettuce 

 and other vegetables, and severely injure gooseberries and 

 other small fruits. 



The rusts, so-called because they appear as streaks or 

 patches of yellow or black color on leaves and other parts of 

 some plants, are very destructive. Rust of the cereal grains, 

 particularly oats and wheat, causes damage in this country 

 to the amount of more than fifteen million dollars every 

 year. The rusty yellow-brown spots seen on wheat leaves 

 in early summer are masses of spores from mycelia in the 

 tissue of the leaf. These spores are distributed by wind, 

 and thus infect other plants in the same field. The food- 

 supply of the rust is drawn from the sap of the wheat plant, 

 with the result that food for the proper formation of grain 

 is not available. Some kinds of rusts require two plants 

 as hosts during their life-history. As an example, one kind 

 of wheat rust has spores that germinate on barberry leaves 

 in the spring, and on the barberry plant the rust produces 

 special spores which infect wheat plants. Of course, this 

 is not the only kind of wheat rust, for in many parts of this 

 country barberry bushes are rarely found. Wheat may 

 suffer from rusts which require no other plant for completing 

 their life-history. 



Smuts. Everybody who has cultivated Indian corn 

 must have noticed the " smutty " ears and tassels, on which 



